The Magnitude 4.5 Earthquake in the Karst Area of Guizhou on 21 August 2021: An Extremely Shallow Earthquake with a Closing-Crack Source

Guizhou province is located in a karst development area in China, and there are many moderate earthquakes, among which extremely shallow earthquakes are likely to cause damage to buildings, transportation lines, and other engineering facilities. Accurate focal depth and moment tensor can provide vital information for earthquake disaster assessment and karst collapse monitoring in karst areas. The network in the study area is relatively sparse, and the velocity structure is complex. For shallow earthquakes, the shallow velocity structure may cause certain errors in the inversion of the focal depth and mechanisms. In this study, based on the surface-wave information of regional sta-tions, the shallow velocity structure is inverted. Based on the updated velocity model, we obtain the focal depth and mechanism of the Mw 4.5 earthquake on 21 August 2021 in Guizhou province. The results show that the optimal focal depth is about 2 km, and the moment of horizontal closing crack (Mcrack) accounts for about 31% of the full moment tensor. The first motions of the P wave at the near-seismic stations of the earth-quake are negative, similar to the typical collapses. However, compared to mining col-lapse earthquakes, the proportion of closing-crake components is relatively low. The dominant double-couple source of the earthquake is a thrust fault, which is consistent with the distribution of faults in the study area. It is speculated that a collapse in the karst area, resulting in negative P-wave polarities and about 31% closing-crake source, induced the shallow earthquake.

Automated summary

Guizhou province in China is prone to moderate earthquakes, especially extremely shallow earthquakes that can cause damage to infrastructure. Accurate focal depth and moment tensor information is crucial for earthquake disaster assessment and karst collapse monitoring in karst areas. However, the sparse network and complex velocity structure in the study area can introduce errors in the inversion of focal depth and mechanisms for shallow earthquakes. This study used surface-wave information from regional stations to infer the shallow velocity structure and obtained the focal depth and mechanism of a 4.5 magnitude earthquake in Guizhou province.